Method and apparatus for evaluation and inspection of composite-repaired structures

ABSTRACT

A method and apparatus for evaluation and inspection of a composite-repaired structure generates a frequency-varying electrical signal to test and evaluate the composite-repaired area. The electrical signal is converted into a mechanical signal and transmitted through the composite-repaired area of the structure. The transmitted mechanical signal is received and converted into an electrical signal for processing. The processed signal is compared with a baseline reference signal to determine whether the composite-repaired area is damaged or undamaged. The baseline reference signal is obtained at the time of the composite repair of the structure.

This is a continuation of application Ser. No. 08/612,421 filed Mar. 7,1996 now U.S. Pat. No. 5,665,913.

TECHNICAL FIELD

The present invention relates to an apparatus and method for evaluationand inspection of structures and, in particular, to an apparatus andmethod for non-destructive evaluation and inspection ofcomposite-repaired metal structures.

BACKGROUND OF THE INVENTION

In the aerospace industry, as well as other industries, utilizingcritically important structural members, repair of damaged structuralmembers is increasingly being performed using composite repair.Composite repair refers to the repair of a damaged structure (e.g., analuminum aircraft wing panel having a damaged area) by adhesivelybonding a composite material, such as a multiple ply composite material,to the damaged structure.

After the structure has been composite-repaired and is put back intoservice, the integrity of the composite-repaired structure mustgenerally be monitored. Periodically, these composite-repairedstructures undergo non-destructive evaluation and inspection (NDE/I) toensure the composite repair is not damaged or otherwise failing.

Conventional NDE/I techniques utilize eddy-current, ultrasound, thermalimaging, laser, X-ray, etc. All of these techniques require substantialaccessibility to the structure to be evaluated and inspected. Inaircraft structure evaluation and inspection, for example, performanceof conventional NDE/I requires disassembly of the aircraft structure(s)to gain access to the inspection article. In some cases ofcomposite-repaired structural inspection, such as inspection of a C-130outer wing fuel tank, it requires upwards of 1300 man-hours todisassemble the aircraft structure to gain access to the inspectionarticle for conventional NDE/I.

Accordingly, there exists a need for an apparatus and method fornon-destructive evaluation and inspection of a repaired structure thatreduces or eliminates the need for substantial accessibility in order tomonitor, evaluate and/or inspect the repaired structure. Further, thereexists a need for an apparatus and method for NDE/I of intactcomposite-repaired structures, thus reducing or eliminating the need fordisassembly of the composite-repaired structure or article from theoverall structure.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an apparatus andmethod for non-destructive evaluation and inspection of a repaired areaof a structure. The present invention includes a signal generator forgenerating a first electrical signal. A transducer coupled to the signalgenerator and to the structure converts the first electrical signal intoa mechanical signal for transmission through the repaired area of thestructure. Another transducer coupled to the structure receives andconverts the mechanical signal into a second electrical signal. Theapparatus further includes a signal processor coupled to the secondtransducer for generating, in response to the second electrical signal,an output signal indicative of the present condition of the repairedarea of the structure. The output signal is then used for comparison toa baseline reference signal to determine whether the repaired area ofthe structure is damaged. The baseline reference signal is generatedafter repair of the structure is accomplished.

In an alternative embodiment of the present invention, the signalprocessor includes an impedance analyzer for measuring the impedanceresponse of the repaired area in relation to the frequency of the firstelectrical signal generated by the signal generator. The impedanceresponse measured at the time of testing is compared with the impedanceresponse measured at the time of repair of the structure (undamagedcondition) to determine if the repaired area is damaged.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is made to the following detaileddescription taken in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates a block diagram of an apparatus for non-destructiveevaluation and inspection of a repaired structure in accordance with thepresent invention;

FIG. 2 illustrates an electromechanical diagram of the present inventionand illustrates a more detailed block diagram of a signal processor usedin accordance with the present invention;

FIG. 3 shows an alternative embodiment of the present inventionillustrating the use of an impedance analyzer;

FIG. 4 illustrates two impedance-vs-frequency signal plots for anundamaged repaired area and a corresponding damaged repaired area; and

FIG. 5 illustrates a method of non-destructive evaluation and testing ofa repaired area of a structure in accordance with the present invention.

DETAILED DESCRIPTION

With reference to the drawings, like reference characters designate likeor similar parts throughout the drawings.

With reference to FIG. 1, there is shown a non-destructive evaluationand inspection tester 10 in accordance with the present invention. Thetester 10 includes a signal generator 12, a first transducer 16, asecond transducer 18 and a signal processor 22. The signal generator 12generates and outputs a frequency-varying test electrical signal 14having voltage=v * sin(wt) and current=i * sin (wt+q), where v and i arethe amplitude of the voltage and current, respectively. In the preferredembodiment, the signal generator 12 is a sine wave generator and thetest signal 14 varies in frequency over time, such as a sine sweep overa predetermined frequency range.

Also in FIG. 1, there is shown a composite-repaired structure 30 havinga composite repair doubler 32 including a composite-repaired area 31.Typically, the structure 30 is a metal structure such as an aluminumplate. Generally, the composite repair doubler 32 is used to repair adamaged area of the structure 30 by having the composite repair doubleradhesively bonded onto the structure 30 to repair the damaged area. Thecomposite repair doubler 32 covers the composite-repaired area 31. It isthe integrity of the composite repair doubler 32 or area 31 that istested by the present invention.

The first transducer 16 is coupled to, or installed onto, the structure30 near the composite repair doubler 32, as shown. The first transducer16 is also electrically coupled to the signal generator 12 and receivesthe signal 14. The first transducer 16 converts the signal 14 into amechanical signal and transmits the mechanical signal into the compositerepair area 31 and through both the structure 30 and composite repairdoubler 32.

The second transducer 18 is also coupled to, or installed onto, thestructure near the composite repair doubler 32, as shown. As will beappreciated, the transducers 16, 18 can be located in any configuration,with a preferred configuration such that an imaginary line drawn betweenthe two transducers 16, 18 intersects a point on the composite repairdoubler 32, i.e. in the area 31. In the preferred embodiment, thetransducers 16, 18 are positioned diagonal to the corners of the doubler32, as shown, such that the imaginary line drawn between the traducers16, 18 substantially intersects with the central region of the compositerepair doubler 32 (area 31).

The mechanical signal produced by the transducer 16 travels through boththe structure 30 and the composite repair doubler 32 (area 31) and isreceived by the transducer 18. The transducer 18 converts the receivedmechanical signal into an electrical signal and outputs a responseelectrical signal 20. In the preferred embodiment, the transducers 16,18 are piezoelectric transducers (PZT) whereby the first transducer 16is a signal transmitter PZT and the second transducer 18 is a sensorPZT.

After output from the second transducer 18, the signal 20 is input tothe signal processor 22 for processing to generate an output signalindicative of the present condition of the composite-repaired structure30. Damage detection of the composite-repaired structure 30 (i.e. damageof the composite repair doubler 32 in the form of delamination, disband,crack propagation, etc.; and increased damage to the structure 30 suchas crack propagation, etc.) is accomplished by comparing the presentoutput signal from the transducer 18 to a baseline reference signalgenerated at the time of composite repair and stored in the signalprocessor 22 for subsequent comparison.

As will be understood, the baseline reference signal(s), representingthe undamaged condition of the composite-repaired structure 30, isgenerated at the time of the composite repair of the structure 30. Thebaseline reference signal(s) is then stored for later use when thecomposite-repaired structure 30 is tested and inspected to determine theintegrity (damaged or undamaged) of the repair after thecomposite-repaired structure 30 has been in use for some period of time.

Now referring to FIG. 2, there is illustrated an electro-mechanicaldiagram of the present invention and a more detailed block diagram ofthe signal processor 22. The signal processor 22 includes an isolationfilter and amplifier 40, a frequency domain integrator 42, and a digitalindicator 44. The response electrical signal 20 (the current testsignal) output from the second transducer 18 is input to the filter andamplifier 40. The filter and amplifier 40 filters out any signals havingfrequencies outside the range of frequencies of the test electricalsignal 14 generated by the signal generator 12, and further amplifiesthe filtered signal. The filtered signal, referred to as the sine sweepresponse 41, is input to the frequency domain integrator 42. Thefrequency domain integrator 42 generates and outputs a DC signal 43 inresponse to the sine sweep response 41. The digital indicator 44receives the DC signal 43 and provides a display (or value) indicativeof the present condition of the composite-repaired structure 30. Thedisplay is thereafter read or stored by the user for comparison with thebaseline reference signal(s).

As illustrated in FIG. 2, the composite repair doubler 32 possessesparticular electromechanical admittance and/or impedancecharacteristics. These characteristics are determined by the physicaland/or chemical properties of the materials of the composite repairdoubler 32. These properties include inertia, spring and viscous dampingdesignated by coefficients m, k and c, respectively, with the mechanicalproperties analogous to the electrical properties of inductance,capacitance and resistance. A change in the physical and/or chemicalcharacteristics of the composite repair doubler 32 causes acorresponding change in the coefficients m, k or c. It is this change(difference measured at two different times) that is used to detect thepresence of a damaged condition (delamination, disbond, crackpropagation, etc.) of the composite repair doubler 32.

Now referring to FIG. 3, there is shown an alternative embodiment of atester 100 in accordance with the present invention. In replacement ofthe signal processor 22 in the tester 10, an impedance analyzer 102 iscoupled to the test electrical signal 14 and the response electricalsignal 20. The impedance analyzer 102 measures the impedance and/oradmittance from the first transducer 16 to the second transducer 18. Aswill be understood, a damaged condition in the composite repair doubler32 will result in an impedance response different from the impedanceresponse (baseline reference signal) generated from an undamagedcomposite repair doubler 32. Preferably, the impedance analyzer 102 is aHewlett-Packard HP 4194A impedance analyzer capable of providingimpedance signal response (magnitude and phase) graphs or plots inrelation to frequency (of the test electrical signal 14). As will beappreciated, any measuring equipment, and the like, capable of measuringany change(s) in the properties of the composite repair doubler 32detected in response to the signal(s) passing through the compositerepair doubler 32 may be used.

The following provides an example of the detection of a damagedcomposite repair doubler 32 in accordance with the present invention: A14-ply B/Epoxy composite repair doubler 32 (7.5 inch length, 2.5 inchwidth, 0.1 inch thickness) was applied to an aluminum plate specimen 30.During application, a thin metallic shim (1.25 inch length, 0.5 inchwidth, 0.05 inch thickness) was inserted between the composite repairdoubler 32 and the aluminum plate specimen 30.

After the composite repair doubler 32 was cured, the test specimen wastested using the tester 10 in accordance with the test configurationshown in FIG. 1. The digital readout from the signal processor 22provided an output signal of "68". Further, the test specimen was testedusing the tester 100 in accordance with the test configuration shown inFIG. 3 whereby impedance-vs-frequency plots or graphs were obtained fromthe impedance analyzer 102.

After the baseline signals from the above testing of the undamagedcomposite doubler 32 were generated, the thin metallic shim was removedfrom the composite repair doubler 32 to provide a damaged condition(disbond) for the composite repair doubler 32. Using the testconfiguration shown in FIG. 1, the output signal from the tester 10provided a reading of "92". An output signal of "68" was generated bythe tester 10 for an undamaged condition and an output signal of "92"for a damaged condition.

Likewise, impedance-vs-frequency plots or graphs were generated from theimpedance analyzer 102 for the damaged condition. The graphs generatedfrom the undamaged and damaged composite repair doubler were overlayedand the resulting graphs 200, 202 are shown in FIG. 4. The solid linerepresents measurements for the undamaged condition and the dotted linerepresents measurements for the damaged condition. Clearly, there isdetected a distinct difference between the undamaged and damagedconditions.

Now referring to FIG. 5, there is shown a method 300 for non-destructiveevaluation and testing of a repaired area of a structure in accordancewith the present invention. At a step 302, the signal generator 12generates the test electrical signal 14 for input to the firsttransducer 16. In the preferred embodiment, the frequency of the testelectrical signal 14 varies according to a desired pattern (e.g.frequency sweeping from 18 KHz to 19 KHz over a time period T). As willbe appreciated, the signal 14 may have a single frequency or a range offrequencies. At a step 304, the electrical signal 14 is converted into amechanical signal. The mechanical signal is transmitted through thecomposite-repaired doubler 32 (i.e. the repaired area 31) at a step 306.

After transmission through the repaired area 31, the mechanical signalis received by the second transducer 18 at a step 308. At a step 310,the received mechanical signal is converted into the response electricalsignal 20. The signal 20 is processed to generate a display signal at astep 312. At a step 314, the output signal is compared to a baselinereference signal to determine whether the composite-repaired doubler(the repaired area 31) of the structure 30 is damaged. As will beappreciated, an alternative embodiment of the method 300 includesrepeating the steps 302 through 312 after the structure 30 is repairedwith the composite repair doubler 32. Performance of these steps afterrepair of the structure will generate the baseline reference signal thatis used, at step 314, for comparison to the output signal to determineif a damaged condition is present.

Although several embodiments of the present invention has been describedin the foregoing detailed description and illustrated in theaccompanying drawings, it will be understood by those skilled in the artthat the invention is not limited to the embodiments disclosed but iscapable of numerous rearrangements, substitutions and modificationswithout departing from the spirit of the invention.

What is claimed is:
 1. An apparatus for testing and inspection of acomposite-repaired area of a structure, comprising:a signal generatorhaving a test signal output; a first transducer attached to thestructure and receiving the test signal from the signal generator forconverting the test signal to a mechanical signal transmitted through aportion of the composite-repaired area of the structure; a secondtransducer attached to the structure for receiving the mechanical signaltransmitted through the structure and converting the received mechanicalsignal into a response signal varying with the mechanical properties ofthe composite-repaired area; and a signal processor coupled to thesecond transducer and receiving the response signal to output a displaysignal indicative of the mechanical properties of the composite-repairedarea of the structure, said signal processor comparing the displaysignal to a baseline reference signal representing the mechanicalcondition of the composite-repaired area.
 2. The apparatus in accordancewith claim 1 wherein the signal processor outputs a display signalindicative of the mechanical properties of the composite-repaired areaincluding inertia, spring damping and viscous damping.
 3. The apparatusin accordance with claim 1 a wherein the response signal varies inaccordance with the chemical characteristics of the composite-repairedarea.
 4. The apparatus in accordance with claim 1 wherein said secondtransducer responds to the mechanical properties of thecomposite-repaired area to output an electrical signal includinginductance, capacitance and resistance.
 5. An apparatus for testing andinspection of a composite-repaired area of a structure, comprising:asignal generator having a test signal output; a first transducerattached to the structure and receiving the test signal from the signalgenerator for converting the test signal to a mechanical signaltransmitted through a portion of the composite-repaired area of thestructure; a second transducer attached to the structure for receivingthe mechanical signal transmitted through the structure and convertingthe received mechanical signal into a response signal varying with theinertia, spring damping and viscous damping of the composite-repairedarea; and an impedance analyzer coupled to receive the response signaland the test signal for measuring the impedance and/or admittance fromthe first transducer to the second transducer and generating animpedance versus frequency display signal varying with the inertia,spring damping and viscous damping of the composite-repaired area. 6.The apparatus in accordance with claim 5 wherein said second transducerresponds to the inertia, spring damping and viscous damping of thecomposite-repaired area to convert the mechanical signal into theanalogous electrical properties of inductance, capacitance andresistance, respectively.
 7. The apparatus in accordance with claim 6wherein said impedance analyzer responds to the inductance, capacitanceand resistance measurements as represented in the response signal.
 8. Amethod for evaluation and testing a composite-repaired area, where abaseline reference signal representing a repaired condition of thecomposite-repaired area was previously generated, comprising the stepsof:transmitting a signal through the composite-repaired area of thestructure; receiving the signal transmitted through thecomposite-repaired area; generating an output signal in response toreceiving the signal, said output signal varying with the properties ofthe composite-repaired area including inertia, spring damping andviscous damping; and comparing the output signal to the baselinereference signal to determine a change in the properties of thecomposite-repaired area of the structure.
 9. The method in accordancewith claim 8 wherein the step of transmitting the signal includes thesteps of:generating a first frequency-varying electrical signal;converting the first electrical signal into a mechanical signal; andtransmitting, at a first location on the structure, the mechanicalsignal through a portion of the composite-repaired area of thestructure.
 10. The method in accordance with claim 9 wherein the step ofreceiving the transmitted signal includes the steps of:receiving, at asecond location on the structure, the mechanical signal transmittedthrough the composite-repaired area; and converting the receivedmechanical signal into a second electrical signal.
 11. The method ofdetermining a baseline reference signal and an output signal for testingand inspection of a composite-repaired area of a structure, comprisingthe steps of:generating a first frequency-varying electrical signal;converting the first electrical signal into a mechanical signal;transmitting, at a first location on the structure, the mechanicalsignal through a portion of the composite-repaired area of thestructure; receiving, at a second location on the structure, themechanical signal transmitted through the composite-repaired area;converting the received mechanical signal into a second electricalsignal; and processing the second electrical signal to generate thebaseline reference signal varying with the mechanical properties of thecomposite-repaired area.
 12. The method in accordance with claim 11wherein the step of processing includes the steps of:filtering out fromthe second electrical signal a range of signals outside the frequency ofthe transmitted signal to generate a sine sweep response; integratingthe sine sweep response to generate either the baseline referencesignal.
 13. The method in accordance with claim 11 wherein the step ofconverting the received mechanical signal into a second electricalsignal includes converting the mechanical signal representing theproperties of the composite-repaired area, including inertia, springdamping and viscous damping into analogous electrical properties ofinductance, capacitance and resistance, respectively.
 14. The method inaccordance with claim 11 wherein the step of converting the firstelectrical signal into a mechanical signal comprises positioning a firsttransducer along a diagonal through the composite-repaired area, andwherein the step of converting the received mechanical signal into asecond electrical signal includes positioning a second transducer alongthe same diagonal as the first transducer.